SBM-20 Tube Specs

The SBM-20 is one of the most popular GM tubes for hobby Geiger Counter and radiation monitoring projects. It is relatively sensitive and easy to use.

There are many example circuits online that will help you build a Geiger Counter using this tube.

One example is the MightyOhm Geiger Counter Kit:

The SBM-20 is most sensitive to hard beta and gamma radiation. It is important to note that due to the thick walls of the tube, the SBM-20 is NOT sensitive to alpha radiation (the walls block the alpha particles), so it is not a good choice for samples that primarily emit alpha radiation, such as Americium. However, it will still detect some of the decay products (those that emit beta or gamma radiation).

Please note that the thin walls are easily damaged. Handle the tube gently to avoid crushing it. Do not drop the tube.

Please consider your safety and the safety of others when using this tube. The tube requires high voltages to operate (approx. 400V) which may pose a risk of electric shock. Also, exposure to radiation, even in small amounts, may be harmful to your health.

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General Specifications (see notes below)

Parameter Units Specification
Overall Length mm 108 ± 3.5
Overall Diameter mm 10 ± 1
Weight grams 10 (nominal)
Operating Temperature Range °C -50 to +70
Lifetime pulses (min) 2e10
Nominal Working Voltage volts 400
Operating Voltage Range volts 350 – 475
Recommended Anode Resistor ohms 5.1M (5.1e6)
Impulse Amplitude volts 50 (min)
Maximum tube current amps 20u (20e-6)
Tube Capacitance pF 4.2

Keep in mind that these tubes are “new old stock” and most have been in storage for some time. They may not meet all of the original manufacturer’s specifications.

In addition, the specifications above have been collected from several sources (there is no single definitive datasheet) and may be subject to change. There are more specs here:

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